Precision Electroweak measurements at the FCC-ee

TL;DR

The FCC-ee will vastly improve electroweak precision measurements across various energies, enabling new physics searches and more accurate fundamental constants, thanks to its high luminosity and advanced energy calibration methods.

Contribution

This paper details the expected precision improvements in electroweak measurements at the FCC-ee, highlighting its potential to significantly advance particle physics knowledge.

Findings

Z boson mass and width measured to better than 100 keV

Weak mixing angle determined with a factor of 30 improvement

W mass measured to 300 keV precision

Abstract

Because of a luminosity of up to five orders of magnitude larger than at LEP, electroweak precision measurements at the FCC-ee -- the Future Circular Collider with electron-positron beams -- would provide improvements by orders of magnitude over the present status and constitute a broad search for the existence of new, weakly interacting particles up to very high energy scales. The FCC-ee will address centre-of-mass energies ranging from below the Z pole to the $\mathrm{t\bar{t}}$ threshold and above. At energies around the Z pole, the Z-boson mass and width can be measured to better than 100 keV each. Asymmetry measurements at the Z pole allow improvements in the determination of the weak mixing angle by at least a factor 30 to $\delta\sin^2\theta\mathrm{_W^{eff}}\simeq 6\times 10^{-6}$. A determination of the electromagnetic coupling constant at the Z energy scale, $\alpha_\mathrm{QED}(m_\mathrm{Z}^2)$, to a relative precision of $3\times 10^{-5}$ can be obtained via measurement of the forward-backward asymmetry of lepton pairs at two energy points $\pm 3.2\,\textrm{GeV}$ away from the Z peak. At energies around the WW threshold, high-statistic cross section measurements can provide a determination of the W mass to 300 keV. The key breakthrough advantage of the FCC-ee in these achievements, beside the large luminosity, is the possibility of a continous, precise determination of the beam energy by resonant depolarization at the Z peak and at the WW threshold. Precise measurements of the hadronic branching fractions of the Z and W bosons allow for considerably improvements in the determination of the strong coupling constant down to a precision of $\delta\alpha_\mathrm{s}(m_\mathrm{Z}^2)\simeq 0.0001$. An energy scan around the 350 GeV $\mathrm{t\bar{t}}$ threshold allows a 10 MeV measurement of the top-quark mass.